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Single Solvent Recrystallization
Example of a Typical Experiment
FAQ: How many boiling stones should I use?
FAQ: How long does it take for the crystals to grow?
FAQ: If we add too much solvent do we just boil it off?
FAQ: My sample has dissolved, but my solvent is just hot. Do I have to wait until it boils?
FAQ: Can I put my hot solution directly into the ice bath?
FAQ: I have a really lousy suction from that water aspirator. What can I do?
FAQ: What should I put on the label when handing in my sample?
FAQ: Can we add the second solvent first? In the end, all are together anyway.
FAQ: If I can choose between the 1-solvent or 2-solvent method, which one should I choose?
Recrystallization is a purification technique.
It works because: 1) different substances have different solubilities in the same solvent, and 2) only molecules of the same compound will fit easily into the crystal lattice of that compound. Impurities remain in solution or stick on the outside of the crystal lattice.
In practice you purify by slowly cooling a hot, saturated solution of your compound. While cooling, molecules of the same type align in a crystal lattice, forming crystals. After cooling, crystals are collected by vacuum filtration and washed by rinsing with ice-cold solvent.
Either the one-solvent or the two-solvent method for recrystallization can be used:
Single-solvent method:
The chosen recrystallization solvent will dissolve the compound when hot, but not at room temperature.
Two-solvent method:
The first recrystallization solvent will dissolve the compound at all temperatures. The second solvent will not dissolve the compound at any temperature. The two solvents must be miscible, i.e., soluble in one another, forming a single layer solution.
Insoluble impurities can be filtered by hot gravity filtration.
Decolorization is dealt with by adding decolorizing charcoal (Norit) and then performing a hot gravity filtration.
Single Solvent Recrystallization
Single solvent recrystallization is the most basic and commonly used recrystallization method.
An ideal solvent does NOT dissolve the solid at room temperature BUT dissolves the solid well in hot solvent.
Process:
1. Use solubility tests to determine a suitable recrystallization solvent.
2. Heat the solvent and add a minimum of the hot solvent to your crude product to dissolve it (dropwise addition).
3. Hot gravity filter the hot solution if impurities are present. If your solution is colored, use decolorizing charcoal and then hot gravity filter.
4. Allow the hot, clear solution to slowly cool to room temperature (or 0 oC using an ice bath, if necessary). If crystallization does not occur, induce crystallization.
5. Collect crystals by vacuum filtration and wash the crystals using a minimal amount of cold solvent.
6. Allow the crystals to dry.
Two solvent recrystallization is an alternative and very useful recrystallization method to single solvent recrystallization.
The first solvent should dissolve your crude product very well at room temperature (or in hot solvent). The second solvent should NOT dissolve your crude product at room temperature or in hot solvent.
The two solvents should be completely miscible and preferably have similar boiling points.
Process:
1. Use solubility tests to determine a suitable recrystallization solvent.
2. Heat the first solvent and add a minimum of the hot solvent to your crude product to dissolve it (dropwise addition).
3. Hot gravity filter the hot solution if impurities are present. If your solution is colored, use decolorizing charcoal and then hot gravity filter.
4. Add the second solvent slowly (with shaking) until the solution remains cloudy. Add one or two drops of the hot first solvent until the solution goes clear again.
5. Allow the hot, clear solution to slowly cool to room temperature (or 0 oC using an ice bath, if necessary). If crystallization does not occur, induce crystallization.
6. Collect crystals by vacuum filtration and wash the crystals using a minimal amount of cold solvent.
7. Allow the crystals to dry.
Crystal Line was working with her partner Bea Kurr to purify salicylic acid. They tested the solubility of this solid in several solvents both at room temperature and at the boiling point of the solvent.
Crystal observed that the solid was insoluble in water at room temperature, but soluble in hot water. She also noted that the solid was insoluble in ethyl acetate at room temperature. After discussing these observations with Bea, Crystal decided to use the single solvent recrystallization method since the solvents which might be used in the two solvent method are not miscible and thus not suitable.
In an Erlenmeyer flask Crystal dissolved about 1 g of the solid in about 5 mL of hot water by heating on a hot plate with swirling to make a fine slurry. After about 1 minute, solid remained and thus she added another 4 mL of hot solvent in portions, with swirling and heating. She continued to add hot water in 2 mL portions with swirling and heating every minute or so until all of the solid dissolved (total of 13 mL of solvent).
Since the solution was not highly coloured, Crystal and Bea decided it was not necessary to decolourize it. However, Crystal noted a few insoluble coloured granules in the solution. Thus, she performed hot gravity filtration. She poured the hot solution into a fluted filter paper contained in a hot powder funnel. The receiving Erlenmeyer flask was covered by a beaker and kept hot by heating it in a steam bath.
The filtrate was removed from the steam bath and allowed to cool to room temperature. Crystals of pure salicylic acid slowly began to appear in the flask. The flask and contents were cooled further in an ice-water bath for about 20 minutes.
Bea set up the vacuum filtration apparatus: a Buchner funnel on top of a filter flask connected to a vacuum trap apparatus. The ice-cold flask and contents were swirled and poured onto the Buchner funnel under reduced pressure. After all crystals were in the funnel, Bea released the pressure and washed the crystals with a little bit of ice-cold solvent. Then she put the vacuum on again.
When no more water was seen draining from the filter, she placed the crystals on several filter papers and covered the solid with more filter papers, crushing the acid and pressing firmly to remove as much water as possible. The top filter papers were removed and the product set aside to air dry. When the crystals were completely dry, Crystal crushed a small sample on a porous plate to prepare a sample for the melting point.
Make sure, that the solvents you add are boiling or hot!
It is very important that you add the minimum amount of boiling solvent in order to get a saturated solution. If you add too much solvent, the solution may be too dilute for crystals to form.
It is important to slowly cool the flask first to room temperature and then in ice-water. A rushed crystal formation will trap impurities within the crystal lattice. Furthermore, the resulting crystals will be smaller.
If none of the solvents tested is suitable for the single-solvent method, use the two-solvent method for recrystallization. In most cases, the single-solvent method is the recrystallization method of choice.
If no crystals form, try: 1) scratching the inside of the flask with a glass rod at the interface of the solution or 2) concentrating your solution by boiling off some solvent. You may have too much solvent, i.e., your solution is not saturated, or 3) try the two-solvent recrystallization method.
Do not move the flask during the crystal formation phase. Disturbing it can lead to the formation of small crystals and the incorporation of impurities in the crystal lattice.
Use the water aspirator as a vacuum source in preference to the house vacuum line, because fumes and gases will dissolve in the water and be diluted and disposed of. The house vacuum line may be used if the water aspirator produces very little vacuum and no noxious gas has been involved in your previous experimental steps.
When collecting crystals by vacuum filtration, release the vacuum, rinse your crystals with a little ice-cold solvent, then reapply the vacuum to remove impurities that might stick to the crystals.
FAQ's
Q: The solvent that we use to dissolve the sample for TLC, is that the solvent we will use for recrystallization?
This is not necessarily so, but it could be the case. Select a suitable recrystallization solvent by testing the solubility of your unknown solid sample in different hot and cold test solvents. After noting the solubility properties of the solid, you can choose the appropriate recrystallization method.
Q: How many boiling stones should I use?
You want to use 1 or 2 boiling stones for about every 100 mL of liquid. Remember to remove them after recrystallization!
Q: When I tested the recrystallization solvent in a test tube it worked, but now my sample won't dissolve!
Did you have water in your test tube? Did you use the proper ratio of solid (0.1 g) to solvent (1 mL)?
Q: How long does it take for the crystals to grow?
It is impossible to know exactly how long it will take for the crystals to form.
Until you have a little more experience, a good rule of thumb is to wait until your flask has slowly cooled to room temperature (touch the flask to check temperature). If no crystals have formed by then, try to induce crystallization by scratching the inner side of your flask at the interface of the solution with a glass rod and wait a few more minutes. When small crystals appear,cool your solution on ice about 15 minutes more. Your crystals should be formed by then.
The most important factor affecting recrystallization time is ensuring that you have a saturated solution, obtained by adding the minimum amount of hot solvent to dissolve your crude solid. If you think you might have used too much solvent, you can concentrate your solution by boiling off some of your solvent.
Q: So - once the solvent has reached its boiling point and my crude solid is NOT dissolved, do I add more solvent or do I let it boil longer?
Well, let's think about this. You can let it boil for a moment, but do not wait too long since. The boiling solvent will slowly evaporate, reducing the total volume of solvent added. This means that you need to add even more solvent the next time!
The best method is to add hot solvent in timed intervals. This means, once you have added hot solvent, bring the solution to a boil, then wait about 20 seconds. If your crude solid has not dissolved, add more hot solvent, boil again, then wait another 20 seconds. Continue this process until all of your crude solid is dissolved.
Q: If we add too much solvent, do we just boil it off?
Yes, you should reduce the overall volume by boiling off the excess solvent. Reduce the volume until you find that just a little more solvent needs to be added to completely dissolve the crude solid. By this process, you will obtain a saturated solution.
If you have a large amount of excess solvent, you can speed up the process of boiling off the solvent by holding a side arm test tube over your Erlenmeyer flask. Connect the side arm of the test tube to a vacuum source, preferably the water aspirator or house vacuum. This process helps remove solvent vapors.
Q: My sample has dissolved, but my solvent is just hot. Do I have to wait until it boils?
It is necessary to use hot solvent, but if your solid sample has already dissolved in hot solvent, this is enough. For some compounds, you might need to wait until the solution boils before your compound completely dissolves.
Q: After the solution has cooled down to room temperature, how long should I let it cool in the ice bath?
You should let the solution cool in the ice bath for a minimum of 15 minutes to ensure that the bulk of the sample has recrystallized.
Q: Can I put my hot solution directly into the ice bath?
No. You need to cool the solution first to room temperature before placing it in the ice-water bath. Besides reducing the risk of breaking your flask and loosing your product in the ice-water, you will get better and purer crystals if you let the solution cool slowly.
Q: When we are collecting our crystals using vacuum filtration, what solvent do we use to wash our crystals?
When vacuum filtering, wash your crystals with the solvent you used to recrystallize your compound. However, use ice-cold solvent to ensure that you do not dissolve any of your crystals.
Q: I have a really lousy suction from that water aspirator. What can I do?
Some possible reasons for little suction are:
- The black filter vac adapter between the filtering flask and the Buchner or Hirsch funnel is missing. Make sure it is there and everything is sitting tight
- The water trap is not closed to the atmosphere.
Q: What should I put on the label when handing in my sample?
On your sample label, you should write:
- your name,
- experiment number (e.g., Exp 7),
- the name of the product,
- the product's melting or boiling point,
- the weight of the sample,
- the date.
Q: When using the two-solvent recrystallization method, why is it necessary to keep both solvents hot when adding?
For the single-solvent and the two-solvents recrystallization method it is essential that you prepare a hot, saturated solution. To do this, all solvents must be hot before you add them.
Heating the solvents decreases the kinetic energy necessary to dissolve the compound. This also means that less solvent is needed to dissolve the compound, which makes the solution more saturated. The more saturated the solution, the easier the crystals will form once the solution cools down.
Q: Can we add the second solvent first? In the end, all are together anyway.
No. You must add a minimum amount of first hot solvent to dissolve your compound. Then you decrease the solubility of your solid by adding the second hot solvent to the first cloud. Reheat the solution to clear it again. This allows for a saturated solution and for crystallization to occur upon cooling.
Q: If I can choose between the 1-solvent or 2-solvent method, which one should I choose?
The preferred method of recrystallization uses one solvent. When a suitable single solvent can not be found, the two-solvent method of recrystallization is used.